RISC-V: Split "(a & (1UL << bitno)) ? 0 : 1" to bext + xori

We avoid reassociating "(~(a >> BIT_NO)) & 1" into "((~a) >> BIT_NO) & 1"
by splitting it into a zero-extraction (bext) and an xori.  This both
avoids burning a register on a temporary and generates a sequence that
clearly captures 'extract bit, then invert bit'.

This change improves the previously generated
    srl   a0,a0,a1
    not	  a0,a0
    andi  a0,a0,1
into
    bext  a0,a0,a1
    xori  a0,a0,1

Signed-off-by: Philipp Tomsich <philipp.tomsich@vrull.eu>

gcc/ChangeLog:

	* config/riscv/bitmanip.md: Add split covering
	"(a & (1 << BIT_NO)) ? 0 : 1".

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/zbs-bext.c: Add testcases.
	* gcc.target/riscv/zbs-bexti.c: Add testcases.